Shrink labels fall into two categories: roll-on-shrink-on (“ROSO”) labels and sleeve labels. ROSO labels are supplied from a reel, cut into sheets, applied around a container, and seamed around the container during the labeling step using hot melt to form a seam, with the machine direction (MD) of the film extending circumferentially around the container. ROSO label films primarily shrink in the MD direction and generally employ biaxially oriented polypropylene (BOPP) films.
A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing transparent wrapping which conforms to the contour of the product while providing the usual functions required of packaging materials such as protection of the product from loss of components, pilferage, or damage due to handling and shipment. Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
In certain situations, it is desirable to affect shrinkage along a single axis without substantial shrinkage in the cross-direction. For example, in the process of labeling bottles or cans by shrinking a tube or lap-sealed sleeve of heat shrinkable material, if the film shrinks parallel to the container axis, the label may not be placed in the right position but rather placed above or below the desired position upon shrinkage.
In order to obtain uniaxially shrinkable materials, it is possible to employ uniaxially oriented materials, i.e., materials which are oriented in only one direction. However, uniaxially oriented film can lack the requisite strength and toughness necessary for use in such applications. Inasmuch as biaxially oriented films exhibit desirable strength and tear resistance in both directions of orientation, it would be desirable to obtain a uniaxially heat shrinkable film which is biaxially oriented but substantially stable in the transverse direction (TD). In labeling applications, the shrinkable direction usually corresponds to the MD of the film manufacturing process.
Oriented polypropylene films are particularly desirable, but crystalline polypropylene alone will become very rigid after the normal MD/TD orientation. When going through a second orientation, an oriented polypropylene film will typically break (tear/snap by MD force). On the other hand, if the film starts off (room temperature) too soft it will become too soft even after the second MD orientation, giving low stability. Thus, there is a balance in the level of softness of the polypropylene film that is desirable, but difficult to reach. The inventor, however, has discovered that certain additives and processes can make a biaxially oriented film having desirable properties.
Related disclosures include U.S. Pat. Nos. 7,052,750; 5,691,043; 6,908,687; 6,113,996; U.S. Patent Application Publication Nos. 2011/0268979; 2011/0212338; 2009/0220757; 2008/248229; 2005/214557; and 2003/0068453; EP 1 423 408; EP 1 300 238; and EP 1 632 343; and WO 2009/142805 and WO 2005/097493.